This is my go to website for subways and metros. I use it whenever I plan to visit a city. Its great to understand the overview in each city, but not so convenient when you are actually needing a map while you are trying to find the subway. But this website is amazingly comprehensive:

urbanrail.net > metro - subway - light rail

You can get lost in the web site. Just pick a continent, and then find just about any city with mass transit.